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Pond Inflow and Target Outflow Problem?

The problem occurs in the design configuration step of Pondmaker. The 1, 2, 5, 10, 25, and 50 yr storm events are working properly. In these cases, the pond inflow is greater than the target outflow indicating the need for a basin. However, for the 100yr storm event, this flips and the inflow is less than the target outflow. When this happens, there is an error when the storage estimate is computed and a basin cannot be designed.

The first thing I did was decrease the output increments from 3 min to 1 min. This did help a little, but the inflow is still less than the outflow.

In the attached file, you will see my model with two basins. Basin 10 is the basin with this specific problem. In addition, you will see Basin 9.

Now Basin 9 has a similar problem that I've run into before. Here, all the storms have a pond inflow less than the target inflow. To me, this means that a basin is not necessary for this given situation. I just want to verify that this is true.

Thank you in advance for your help

SWB9,10.zip

  • Hello TimZ,

    The model doesn't seem to have a worksheet filled out for SWB9. However, one issue may be the general setup of the model. In the pre-development scnearios, you have 5 catchments with a combined area of 13.06 acres going into the outfall. In the pond-development scenario, there are 7 catchments with an area of 12.84 acres. The C values are changing as well, but it is possible that the impact of these changes between scenarios results in a peak flow flow in the post-development case being lower than the pre-development case.

    In addition, the target peak for the outfall uses these 5 pre-development catchments, but the outflow from the pond is only trying to match the flow from the upstream catchment ("SWB9 Onsite Detained" for the cast of pond SWB9) to the target outflow. The outflow from SWB9 Onsite Detained is not able to match the outflow from five pre-development catchments making up the target peak value, thus the user notification you are getting is generated.

    You might want to consider adjusting the setup, such combining some of the catchments into a composite catchment or introducing additional outfalls. This may help with the issue.

    For your referenece, here is a TechNote on using PondMaker: http://communities.bentley.com/products/hydraulics___hydrology/w/hydraulics_and_hydrology__wiki/6460.using-pondmaker

    In addition, there is a sample model that has a PondMaker run already filled in. You can find this at C:\Program Files (x86)\Bentley\PondPack8\Samples\Pre and Post Pond Design.ppc.

    Regards,
    Scott

    Answer Verified By: Sushma Choure 

  • Thanks for the info Scott. I will look into it and let you know how things work out.